The present invention relates to a conferencing system and more particularly to such a system for use in a frequency division multiplex (FDM) communications system.
The ability to provide conferencing in a telephone switching system offers increased versatility in regard to system use. As explained in the prior art, a telephone communications system which employs frequency division multiplexing provides added advantages to system subscribers in enabling such subscribers to place calls at any time quickly and directly, while guaranteeing access to thereby enable any subscriber to reach any other subscriber, as the systems described are primarily non-blocking systems.
The prior art pertinent to FDM systems is described in U.S. Pat. No. 3,809,815 issued on May 7, 1974 to Joseph Reed (the inventor herein) et al and entitled COMMUNICATION SYSTEM UTILIZING FREQUENCY DIVISION MULTIPLEXING TO LINK A PLURALITY OF STATIONS EACH CONTAINING A SWITCHABLE SYNTHESIZER and U.S. Pat. No. 3,809,816 which issued to Joseph Reed et al on the same day and entitled COMMUNICATION SYSTEM UTILIZING FREQUENCY DIVISION MULTIPLEXING AND A FREQUENCY PLAN THEREFOR.
These patents describe the use of a digital phase locked loop synthesizer to create a series of frequencies which are utilized in dialing a single sideband communication system to enable operation of the same as a switch telephone system. The apparatus and techniques employed are those of single sideband transmission and reception and have frequency division multiplexing to provide channelization. These systems employ a bandpass filter in the transmitting side so as to provide the necessary inversion of transmitting and receiving frequencies required to establish full duplex communications. Such systems operate to select either an upper or a lower sideband generated in a balanced modulator so that the proper frequency is automatically made available. A predetermined frequency plan serves to accomplish the inversion and maintain the relationship between the intermediate frequency which is used in the transmitting side and the transmission band. This frequency plan is fully described in U.S. Pat. No. 3,809,816.
In any event, these systems have certain limitations in operation. Referring to U.S. Pat. No. 4,107,471 entitled FREQUENCY DIVISION MULTIPLEX COMMUNICATION SYSTEM issued on Aug. 15, 1978 to Joseph Reed and assigned to the Assignee herein, there is shown an improved FDM system which eliminates many of the disadvantages found in the above noted prior art systems. In this system, a receiver IF frequency replaces the direct synchronous detection scheme of the prior art. In this system, two separate intermediate frequencies are employed in such a manner that the difference between transmitting and receiving frequencies thus generated is the difference between the two separate intermediate frequencies, thereby providing the capability of using independent sideband transmission instead of single sideband transmission. This enables additional channel capacity as well as resulting in a great improvement in receiver sensitivity with an increase in transmission efficiency. In this approach, a single synthesizer is employed and a single bandpass filter selects the appropriate upper or lower sidebands of transmission for full duplex operation. Accordingly, one of the two available sidebands is then used for communication, while the other sideband is available for signaling, monitoring, break-in or other telecommunications functions.
As indicated in U.S. Pat. No. 4,107,471, the apparatus depicted and which uses independent sideband transmission, can be adapted to conferencing systems. In any event, such conferencing systems are extremely desirable, but before any such technique can be implemented, one has to provide adequate structure as well as a predetermined operating plan to enable efficient system use.
Conferencing techniques which are employed in present switching systems provide extreme difficulty when implemented. For example, in a conventional telephone system, it is possible to connect three or four subscriber pairs in parallel so as to permit a three or four way conference by utilizing a bridging technique. In such techniques, the subscribers must be located on the same local switch or relatively close to one another. In these techniques, the energy generated by each subscriber's microphone must be shared between a plurality of receivers. The lower energy received at each receiving point serves to limit the number of subscribers permitted in the conference because the received signals have less and less energy as a function of the number of subscribers. Furthermore, the mismatch generated by the bridging circuit further aggravates the losses, thus causing the received signals to be even less audible. It is also understood that if a subscriber is located at a considerable distance from other subscribers to the conference, his apparatus would present a different impedance than the others and hence, he would receive less energy than the others, making the system unacceptable.
In order to circumvent such problems, the prior art employed four wire transmission. In such systems, one wire pair is used by each subscriber to transmit and another wire pair is used by each subscriber to receive. In this manner, an amplifier may be inserted in the receive or transmit paths to enable amplification of each signal so as to make up for the sharing of energy among subscribers. These systems require that a standard two wire circuit be converted to a four wire circuit and then applied to the amplifiers and combined to provide a conferencing function.
Based on practical restraints in regard to amplification and so on, the number of interconnected subscribers in a conference is usually limited to six. In any event, by using conference briges in cascade, a greater number of subscribers can, in fact, be accommodated. In spite of such techniques, these methods still suffer from the fact that subscribers at various distances may require different amplification factors as coupled to the bridge circuits, thus causing deterioration in the ratio of signal to noise.
The techniques as described above are cumbersome and difficult to implement based on a random conferencing scheme. Hence, in such systems, it is usual to use manual operator intervention in order to properly implement a conference between a plurality of subscribers. If automatic operation of conferencing is desired, the system implementation becomes extremely difficult and complex, as will be discerned upon a review of the prior art.
As will be explained, systems employing FDM utilize four wire transmission. In such systems, one frequency is used for transmission, while a second frequency is used for reception. Furthermore, since the transmission is on a single sideband basis, the signals within the cable add in a linear manner and do not interact with each other to form distortion products. In such systems, there are also no carrier signals to cause beats and/or whistles because of slight frequency differences between various transmitters operating at the same nominal frequency.
As will be explained, the effect of these system characteristics is to enable the subscribers to arrange an impromptu conference. However, the extent of the conference is severely limited according to prior art techniques, as will be explained.
It is therefore an object of the present invention to provide an improved system enabling a plurality of subscribers to engage in a conference, while circumventing many of the disadvantages of the prior art, as above explained, and further specifying a conferencing format which enables a great many subscribers to participate without the attendant problems existing in prior art systems.